Electropolishing in organic solutions

ABSTRACT

Electrolyte for electropolishing metal parts, where electrolyte consist of non-hazardous organic as dissolvent and metal salts of the same metal that is to be polished. Such electrolyte can be used for electropolishing stainless steel of various alloy types. Another variation of electrolyte based on non-hazardous organic as dissolvent, is used to electropolish titanium, zirconium, hafnium, tantalum, niobium, molybdenum, and their alloy, for example nitinol. Such electrolyte include potassium, sodium, lithium, or ammonium salts of fluoride-containing salts.

PART I

Electrochemical polishing of metal part comprise in passing electrical current through this metal part which is submerged into bath with electrolyte, and this metal part is connected to positive pole of power source, and the negative pole is connected to special electrode—cathode which is located inside the bath with electrolyte. Due to mainly dissolving picks than valleys, the surface becomes smooth and bright.

This method mostly used in treating corrosion-resistant metals, such as stainless steel, for which electropolishing becomes the final operation. The traditional method of electropolishing stainless steel utilize mixtures of concentrated acids; phosphoric, sulfuric, chromic, and some organic acids. The main drawback of the traditional method: aggressiveness of electrolyte towards workers, equipment; fast changes in electrolyte composition during operation, low quality of polishing stainless steel of ferrite and martensite alloy types. The first and the third drawbacks are obvious, the second on is caused by chemical and electrochemical reactions in acids during electropolishing process:

Reaction on anode is caused by metal Mi dissolving: Mi=M _(i) ^(z) ^(i) ⁺ +z _(i) e ^(−,)  (1)

where M_(i)—metals in stainless steel alloy composition (ferric, chrome, nickel, and others), z_(i)—ion charge of metal M_(i), in which metal is transformed during dissolving on anode, for example for Fe³⁺z_(i)=3, for Ni²⁺z_(i)=2 and so on.

-   -   Reaction of hydrogen reduction is the main reaction on anode:         2H ⁺+2e ⁻ =H ₂,  (2)

However there possible other reactions, such as reduction of ions of metals that dissolve on anode, for example Fe³⁻ +e ⁻=Fe²⁺  (3)

During the reactions (1)-(3) exchange of ions H⁺ in M_(i)z_(i) ⁻ take place during bath exploitation. That means that acids transform gradually into metal salts. These salts accumulate in the electrolyte, partially precipitate, and the electrolyte loose efficiency. Inefficient electrolyte requires utilization and replacement by fresh-made electrolyte. Utilization of old electrolyte is a very costly procedure. Our electropolishing method utilizes solutions of salts of the same metals that are part of chemical compositions of polishing alloy types. For example, in order to polish chrome-nickel stainless steel alloy we can use ferric salts, nickel salts, chrome salts, or only some of these salts. No acids are used in electrolyte, however some amount of acid can be formed during hydrolyze of salts, for example: Fe³⁺+H₂O=(FeOH)²⁺+H⁺  (4) However amount of hydrogen ions H⁺ created according to (4) are significantly lower than in acid solutions, therefore together with reaction (2) on cathode there go the reaction opposite to anode reaction (1) in saline electrolyte: M _(i) ^(z) _(i) ⁺ +z _(i)e⁻ =M _(i),  (5) For example: Fe²⁺+2e ⁻=Fe Ni²⁺+2e ⁻=Ni Cr³⁺+3e ⁻=Cr As a result during electropolishing we can see similar but going to opposite directions reactions (1) and (5) on cathode and anode, the summary result of these reactions is zero, thus composition of electrolyte do not change. Metal that dissolves on anode, is extracted on cathode, and removed from the electrolyte.

The above theoretical scheme of electropolishing and reactions (1) and (5) and can be put into practice if we can select conditions of the process in such way that we do not disturb process of stainless steel smoothening during dissolving on anode. As well known—the main condition of electropolishing process is equality of speed of formation of oxide film and the speed of its dissolving on anode. By switching from acids to salts we significantly reduce (concentration of H⁺), and thus significantly reduce speed of oxide film dissolving. Therefore in order to equalize both reactions we need to reduce speed of formation of oxide film, which can be achieved by switching from water solutions to water-organic solutions, where passivating films are formatted significantly slower. Relatively safe to humans organics, such as ethylene-glycol, ethyl alcohol, glycerin, triethanalomine, duethanolamine, can be used as dissolving agent.

In saline-based electrolytes the cathode reaction (2) takes place together with reaction (5), competing to each other. In order to have more of the reaction (5) instead of reaction (2) it is necessary to: a) select high cathode density, b) select cathode material with high hydrogen overstrain. Also since correspondence of cathode electrical current density to anode current density is inversely proportional to correspondence of cathode area to anode area, it is necessary to have total area of cathode be at least few times lesser that total area of anode (in order to have high current density on cathode).

Metal is precipitated on cathode in form of loose poorly bonded residue. In order to obtain complete mechanical separation of residue from cathode, we use titanium cathodes, since titanium and titanium alloy do not have high hydrogen extrusion overstrain (low speed of reaction (2)), and low bonding with cathode residue.

PART II

Electrochemical Polishing (EP) is processed when metal parts are submerged into ion-conducting solution (electrolyte) and connecting it to positive pole of power source. Smoothing o the surface and brightness happen during dissolving metal in the electrolyte on anode. Polishing occur only when material on the picks dissolve faster than in hollows, and smooth areas remain smooth while dissolving, without etching.

Most of the scientists think that above conditions occur when dissolving on anode happen in passive condition—when surface is covered by passivating film (PF) of oxide nature. This PF is formed when a) electrochemical reaction between metal and water and/or oxygen-containing anions of electrolyte occur, b) products of this reaction dissolve in electrolyte such way that the speed of oxide formation is the same that the speed of oxide dissolving, and the thickness of PF remain the same. If at the same time the speed of PF dissolving is limited by the speed of diffusion of some component of the electrolyte (for example anions or molecules of solver) then mostly picks are dissolved since the speed of diffusion to them is higher then to the hollows. Existence of PH prevent etching of smooth areas of metal surface.

Such metals as Ti, Zr, Hf, V, Nb, Ta, Mo, W form PF very easily, thus in average conditions (ex.: in air or in water) they are always covered by oxide films, for example according to reaction: Ti+2H2O=TiO2+2H2  (1) During anode current the oxide film formation happen electrochemically: Ti+2H2O=TiO2+4H++4e−(2) PF on these metals are very chemically stable and dissolve very slowly even in strong non-organic acid solutions, such as sulfuric, nitric, hydrochloric, phosphoric. Only fluoric acid dissolve them due to formation of complex anions: TiF⁶ ²⁻ , TaF⁷ ²⁻ and similar. Dissolving happen according to reaction: TiO2+4H++6F−=TiF⁶ ²⁻ +2H2O  (3) Such PF features make it difficult to practically electropolish these metals and their alloy. It is necessary to use fluoric acid—source of F-ions. However fluoric acid is very weak, the constant of its dissociation is 6,6·10-4, therefore in order to speed up the reactions similar to (3) it is necessary to add strong acids (sulfuric, phosphoric, etc.) in order to increase concentration of ions of hydrogen (acidity) H+. Thus electrolytes suitable for electropolishing above group of metal contain mixture of strong acids. These are very aggressive solutions, hazardous for health, environment and equipment. Fluoric acid is very volatile, very poisonous and chemically aggressive. Use if such mixtures are restricted in developed countries.

Another principal way to conduct electropolishing is to slow down formation of PF according to reaction (2). It becomes possible by using electrolytes based on organic solvents, for example alcohols (methyl or ethyl), while allowable water concentration is very low. In this case the speed of PF formation according to (2) decrease so much that strong acids, such as sulfuric acid, can be used as electrolyte.

This direction in EP allows creating relatively safe technology of electropolishing, however super complex and expensive. Usually well dehydrated methyl alcohol is used as a solvent; process goes in low than normal temperature, complex precautions measures are set up in order to prevent water to penetrate into electrolyte. This process requires complex equipment, very expensive and low-productive. Besides methyl alcohol is very poisonous.

We suggest new method of electropolishing abovementioned metals and their alloy. This method combines advantages of previous technologies, but lack their deficiencies. The electropolishing method comprise electrolyte based on organic solvent, which is safe or low-hazardous: ethylene-glycol (C2H6O2), tri-ethylene-glycol (C6H14O4), glycerin (CH2OHCHOHCH2OH), polypropylene-carbonate (C4H6O3). Water concentration is not so restricted; depending on alloy type for some electrolytes should not be more than 20%. With such solvents the reaction goes not so fast, therefore as the main component can be selected among less dangerous salts containing fluorine-ions, instead of fluoric acid, such as fluoride (F—), fluoborate (BF⁴ ⁻ ), fluosilicate (SiF⁶ ⁻ ), fluo-aluminate (AlF⁴ ⁻ ). Selection of cation of these salts depends on dissolving requirements of corresponding complex ions of the following type: TiF⁶ ²⁻ , TaF⁷ ²⁻ etc., for example K2TaF7, Li2TiF6, (NH4)2HfF6 in selected solvent.

Particular conditions of EP such as electrolyte composition, temperature, current density, voltage, duration—are selected during experiments and depend on metal and alloy type. Selection of polishing parameters must satisfy the main requirement: speed of PF formation must be equal to speed of PF dissolving.

If speed of PF formation (2) exceed speed of its dissolving, then instead of polishing metal surfaces is covered by oxide layer. It means that chemical activity of electrolyte is not enough, and it is necessary in increase it by raising salt concentration, or temperature, or select more active ions, or decrease water concentration in electrolyte. Based on chemical activity anions are located in the row (from lowest to highest): fluoborate, fluosilicate, fluo-aluminate, fluoride.

On the other hand if speed of PF dissolving is higher than speed of PF formation, then the surface is etched instead of electropolishing, becomes matte. In this case it is necessary to decrease chemical activity of electrolyte: decrease salts concentration, temperature, or select less active anion. 

1. Electrolyte for electropolishing metal parts, comprising water and/or organic dissolvent and metal salts, of the same metal that is to be polished.
 2. Electrolyte for electropolishing titanium, zirconium, hafnium, tantalum, niobium, molybdenum, and their alloy, which comprise organic dissolvent and potassium, sodium, lithium or ammonium salts of fluoride-containing salts.
 3. Electrolyte from claim 1 and 2 where organic dissolvent is selected from a group of non-hazardous organics. 